1. Technical Field
The present disclosure relates to a developing device for use in image forming apparatuses such as copiers, facsimile machines, and printers, and an image forming apparatus and a process cartridge each using the developing device.
2. Description of the Background
In electrophotography, two-component developing methods are widely employed that use a two-component developer comprised of toner particles and magnetic carrier particles. Two-component developing methods have an advantage over one-component developing methods in terms of durability and image quality. A typical two-component developing device includes a developer bearing member containing a magnetic field generator having multiple magnetic poles (hereinafter “developing sleeve”). The developing sleeve is configured to bear a developer on its surface and to convey the developer as it rotates. Japanese Patent Application Publication No. 11-184249 describes a developing device having a developing sleeve containing a magnetic field generator having five magnetic poles. The five magnetic poles include a developer-supplying pole, a pre-developing developer-conveying pole, a developing pole, a developer-separating pole, and a post-developing developer-conveying pole. The developer-supplying pole contributes to supply of the developer to the surface of the developing sleeve. The pre-developing developer-conveying pole contributes to conveyance of the supplied developer to the developing area where the developing sleeve faces a latent image bearing member. The developing pole contributes to development of a latent image in the developing area. The developer-separating pole contributes to separation of the developer from the developing sleeve after the developer has passed through the developing area. The post-developing developer-conveying pole is disposed between the developing pole and the developer-separating pole, and contributes to conveyance of the developer to the position where the developer separates from the developing sleeve after the developer has passed through the developing area. A developer regulator is further disposed facing the developing sleeve between the developer-supplying pole and the pre-developing developer-conveying pole. The developer regulator is adapted to regulate the amount of developer to be conveyed to the developing area. Another two-component developing device has been also proposed further including a developer regulating pole disposed facing the developer regulator and no post-developing developer-conveying pole.
In accordance with recent demand for compact image forming apparatus, the developing device is required to be more compact, and therefore the developing sleeve is also required to have a smaller diameter. However, it may be difficult for a small-diameter developing sleeve to reliably perform the processes of supplying, conveying, and separating the developer and developing latent images. This is because it is difficult for the small-diameter developing sleeve to contain at least five magnets which can generate a magnetic field having a strength enough for performing each process. Generally, the greater the magnetic force of a magnet, the greater the size of the magnet.
Japanese Patent Application Publication No. 2010-204639 describes a more compact developing device having only three magnetic poles.
Such a compact developing device is likely to have a configuration such that the developer is supplied from an upper side of the developing sleeve. The developer supplied from the upper side of the developing sleeve is pressed against the developing sleeve due to its weight. The pressure from the developer is different between an upstream side and a downstream side with respect to a supply screw that supplies the developer to the developing sleeve. At the upstream side, the developer is pressed against the developing sleeve with a higher pressure and therefore the developer forms dense ears on the developing sleeve. By contrast, at the downstream side, the developer is pressed against the developing sleeve with a lower pressure and therefore the developer forms sparse ears on the developing sleeve. As a result, the resulting solid and halftone images may be lacking in uniformity between the upper side and the lower side with respect to the supply screw.